As a social network, Twitter has generated a new way of consuming electoral debates; through the double screen phenomenon, spectators watch the debate while they pay attention to the comments. Consequently, the credibility attributed to candidates gets influenced by the context created on Twitter during the campaign. Credibility, appeal and power constitute the three image factors in political communication; equally, credibility is made of three subfactors: Expert knowledge, ethical trustworthiness and social efficacy. This research describes the credibility contexts which the four main candidates to the presidency of the Spanish Government tried to implement in Twitter, when heading to the 2015 and 2016 general elections. The evolution of the credibility factors is also analyzed, taking two events as references: The debate between Mariano Rajoy and Pedro Sanchez in December 2015; and the June 2016 debate among all four contenders. Equally, the perception frames for the credibility of the candidates among the young public was also analyzed. The context of perception has been defined after studying the polls, organizing focus groups and conducting content analyses of the Twitter profiles of the candidates. In addition, those content analyses registered the variables that candidates applied in order to look credible. While everybody tried to transmit social efficacy, this subfactor received almost no recognition from audiences. The debates did not change the practices on social media, but the spontaneity of Mariano Rajoy was revealed as more credible than the artificial efforts of his adversaries.